In an IGBT (Insulated Gate Bipolar Transistor) an isolated gate FET (Field Effect Transistor) is used for control of a bipolar transistor. In so doing, low-loss and fast voltage control of the isolated gate FET is combined in a single semiconductor device with the high current and low saturation voltage VCEsat of the bipolar transistor. Accordingly, IGBTs are widely used in medium to high-power applications such as switching mode power supplies, inverters and traction motor controls. A single power IGBT may have a current switching capability of up to about 100 A or more and may withstand blocking voltages of up to 6 kV or more. In power applications, modules of several individual IGBTs, which are connected in parallel, can be used to reach current handling capabilities of up to several hundred amperes at high blocking voltage.
Besides blocking capability and low saturation voltage VCEsat, switching speed and softness during switching-off, i.e. soft-recovery behavior, are important characteristics. Softness may be described in terms of overvoltages and/or voltage oscillations and/or current oscillations occurring during switching-off. In many applications these parameters can be desired to be below certain limits. However, a better softness of IGBTs may be accompanied with higher switching losses. In the current-conducting on-state, the drift region of an IGBT is flooded with a plasma of charge carriers (electrons and holes) ensuring a low saturation voltage VCEsat. During switching-off or commutation of the IGBT, the stored charges of the plasma have to be removed again. Recombination of holes and electrons in the drift region is typically of minor importance. Accordingly, there is typically also a trade-off relationship between saturation voltage VCEsat and softness of IGBTs and IGBT modules, respectively.